1. Field of the Invention
The present invention relates to a process for producing a polyester resin comprising a dicarboxylic acid constitutional unit, a diol constitutional unit having a cyclic acetal skeleton, and optionally a diol constitutional unit having no cyclic acetal skeleton.
2. Description of the Prior Art
Polyethylene terephthalate (hereinafter occasionally referred to as “PET”) has been extensively used as films, sheets, hollow containers, etc., because of its excellent transparency, mechanical properties, melt stability, solvent resistance, fragrance retention property and recyclability. However, PET is not necessarily good in heat resistance and, therefore, various modifications are made thereon by copolymerization.
In general, it is known that polymers modified with a compound having a cyclic acetal skeleton are improved in heat resistance, adhesion property, flame retardancy, etc., owing to the rigid structure and acetal bond of the cyclic acetal.
For instance, U.S. Pat. No. 2,945,008 describes that PET modified with 3,9-bis(1,1-dimethyl-2-hydroxyethyl)-2,4,8,10-tetraoxaspiro[5.5]undecane exhibits a high glass transition point and is excellent in heat resistance. Japanese Patent No. 2971942 discloses a container made of a copolyester produced from terephthalic acid, 1,4-butane diol and a glycol having a cyclic acetal skeleton which is excellent in transparency and heat resistance, as well as a method for producing the container. In addition, Japanese Patent No. 1979830 discloses a polyester produced using a diol having a cyclic acetal skeleton as an example for polyesters excellent in heat resistance and transparency.
Regarding the utilization of the adhesion property due to acetal bond, Japanese Patent Nos. 1843892, 1855226 and 1902128 and Japanese Patent Application Laid-Open No. 4-88078 disclose a polyester-based adhesive, adhesive composition and coating agent which are produced using a diol or dicarboxylic acid having a cyclic acetal skeleton.
In addition, polyesters produced using a dicarboxylic acid or diol having a cyclic acetal skeleton are disclosed as a polyester combined filament yarn with shrinkage difference, for example, in Japanese Patent Application Laid-Open No. 3-130425, as a modified polyester film, for example, in Japanese Patent Application Laid-Open No. 8-104742, as a biodegradable polyester, for example, in Japanese Patent Application Laid-Open No. 9-40762, as an electrostatic developing toner, for example, in Japanese Patent No. 1652382, and as a flame retardant resin composition, for example, in Japanese Patent Application Laid-Open No. 2000-344939.
Polyester resins are generally produced by a direct esterification method in which a dicarboxylic acid is reacted with an excess amount of diol to form an ester thereof, and then the resulting ester is polycondensed under reduced pressure to a polyester; or by a transesterification method in which an ester produced from a dicarboxylic acid and an alcohol is transesterified with an excess amount of diol to an ester of the dicarboxylic acid and the diol, and then the resulting ester is polycondensed under reduced pressure to a polyester. The direct esterification method is industrially advantageous in the production of PET as compared to the transesterification method because terephthalic acid is cheaper than dimethyl terephthalate. Further, the direct esterification method requires no catalyst for producing the ester of a dicarboxylic acid and a diol, thereby providing a high-quality polyester containing a less amount of impurities such as catalyst residues. Thus, the direct esterification method is more advantageous as compared to the transesterification method. In addition, the by-product in the formation of the ester of a dicarboxylic acid and a diol is alcohol for the transesterification method, and water for the direct esterification method. Therefore, the direct esterification method is less detrimental to environments as compared to the transesterification method.
However, in the production of a polyester resin containing a diol constitutional unit having a cyclic acetal skeleton by an ordinary direct esterification method, the cyclic acetal skeleton tends to be decomposed by the action of carboxyl groups and water in the reaction system to provide a polyester having a considerably broad molecular weight distribution or a gelated polyester. The moldability, mechanical properties, etc. of such a polyester are much more poor as compared to a corresponding polyester produced by the transesterification method.
Among the above patent documents, only Japanese Patent Application Laid-Open No. 3-130425 is concerned with the direct esterification method, but fails to specifically describe the production conditions and the ring opening reaction of the cyclic acetal skeleton caused by dicarboxylic acid.
Recently, a strong attention has been paid to the recycling of PET, especially to the chemical recycling thereof, and many PET chemical recycling methods have been proposed (for example, Japanese Patent Application Laid-Open Nos. 2002-60543, 2002-60369 and 2002-167469).
The method for producing polyester resins using bis(β-hydroxyethyl) terephthalate (hereinafter occasionally referred to as “BHET”) recovered from PET by chemical recycling method, not only is less detrimental to environments, but also is industrially useful. In addition, BHET is substantially free from the acid end groups derived from the dicarboxylic acid and, therefore, is suitable as a raw material for the production of polyesters containing a diol constitutional unit having a cyclic acetal skeleton. However, any of the above patent documents fail to disclose the method of producing polyesters containing a diol constitutional unit having a cyclic acetal skeleton by using, as a raw material, an ester of a dicarboxylic acid with a diol which is obtained by depolymerizing a polycondensed product.